Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers which are, typically, sulfur curable or sulfur cured as the case may be, diene based elastomers. The tire rubber, including its tread portion, is typically reinforced with carbon black reinforcing filler and with a minimal, if any, of silica.
In one aspect, treads of rubber tires are often prepared of a cap/base construction in which the outer portion of the tread is the cap and the underlying part of the tread between the tread cap and the supporting tire carcass, is its base. The cap portion is usually designed to be ground-contacting and, thus, have associated properties and the base portion generally overlays the tire carcass and is usually designed to support the cap, therefore, not to be ground-contacting. Such cap/base constructions are well known to those skilled in the art.
In one aspect of this invention, a tread of cap/base construction is contemplated in which, for the purposes of this invention, the tread cap is substantially reinforced with silica, with a minimal amount of carbon black, usually less than about 15 phr of carbon black, and the underlying tread base is substantially carbon black reinforced.
In one aspect, the tire tread, or tread base in the case of a cap/base construction, may be presented in a form of a wing extension on each side of the tread in which the tread wings extend outward and over a portion of the outer surface of the tire sidewall of the tire carcass. Such tread wings may sometimes be referred to as "miniwings". Thus, the tire construction, insofar as the tread construction is concerned, is a tread-over-sidewall construction.
It is to be appreciated that conventional tread, tread cap/base and tread wing constructions are typically prepared by an extrusion process where, in the case of a cap/base-wing construction for example, the cap, base and wings are extruded together to form a one piece, unified extrusion. Such tread extrusion processes are well known to those having skill in such art. However, conventionally in a tread-over-sidewall tire construction, the tread wings are of a rubber composition approximately the tire sidewall composition, primarily for tread-to-sidewall compatibility.
In the practice of this invention, usually, the tread cap is of one rubber composition, and the tread base and the tread wings are of individual differing rubber compositions. Indeed, the tread wings are more conventionally of a rubber composition somewhat similar to the sidewall of the tire carcass. The tread cap is typically of a composition designed to be road contacting and thus have appropriate traction, rolling resistance and treadwear characteristics. The tread base may be of a rubber composition, for example, designed to assist the tread cap in enhancing tire rolling resistance but to not be as effective as the tread cap in promoting traction and treadwear because it is not intended that the tread base be normally designed to be road contacting.
For the purposes of this presentation, a tire is viewed as being composed of a circumferential tread and supporting carcass therefor. As hereinbefore discussed, the tread may be composed of a tread cap, tread base and, optionally, tread wings. The carcass is viewed as being composed of relatively conventional elements which include, but are not limited to, electrically conductive carbon black reinforced rubber sidewalls (although a portion of an outer surface of the sidewalls may be colored by an appropriate pigment such as, for example, white titanium dioxide and, thus, not contain carbon black), beads, apex, innerliner and the supporting carcass plies, including fabric reinforced plies. A shoulder region of a tire is considered as including a portion of the tire in which its sidewall meets its tread. It is not normally a sharp line of demarkation and its actual position may vary somewhat from tire to tire. The bead portion of the carcass is typically composed of a relatively inextensible bundle of wires which is encased in carbon black reinforced rubber and is designed to contact a metal rim on which the tire itself is mounted to form a tire/rim assembly which itself is conventionally adapted to be mounted on a vehicle, particularly a wheel of a vehicle. The rim is typically steel or aluminum, or alloy thereof and is, thus, electrically conductive since the metal is considered as having a very low resistance to flow of electricity. The term "metal", as used herein for the metal rim, is intended to mean electrically conductive metals such as, for example, the aforesaid steel and aluminum rims as would be understood by those having skill in such art.
It is acknowledged that, in some tire constructions, carbon black reinforced rubber components such as, for example, components which are sometimes known as chippers and chafers may be positioned in the bead area or region of the tire construction to assist in cushioning the bead component against the metal rim. In the context of this description, a reference to the aforesaid bead component of the tire carcass is intended to include such other associated rubber components unless otherwise indicated, and thereby are a part of the tire carcass.
In practice, as is well known to those having skill in such art, the tire, which may sometimes be referred to as a pneumatic tire, is mounted on the metal rim and air pressure is applied to the cavity enveloped by the metal rim and the pneumatic tire carcass.
The hereinbefore construction elements, or components, of a pneumatic tire and tire carcass, as well as such tire/rim assembly, are also well known to those familiar with such tire art.
It is important to appreciate that uncompounded rubber by itself is generally considered as being, substantially, an electrical insulator or, in other words, a rather poor conductor of electricity.
A carbon black reinforced rubber vehicular tire, while still providing a degree of resistance to flow of electricity, has a considerably higher electrical conductivity, or lower resistance to flow of electricity, than rubber without the carbon black reinforcement.
It is considered herein that a continuous relatively low electrical resistance path is created between the electrically conductive metal rim of a tire/wheel (tire/rim) assembly to the outer tire tread surface, and thence to the ground via the carbon black reinforced rubber of the tire, including its ground-contacting tread component, for such a tire/rim assembly mounted on a vehicle which is intended to travel over the ground.
In this manner, it is considered herein that potential electrical energy, which may potentially be created by components of or within a moving vehicle as its rotating wheels and associated tire/rim assemblies as they travel over the ground, is dissipated from the rim of a tire/rim assembly on the vehicle to the ground via the carbon black reinforced rubber path of the tire carcass and tread, or tread cap of a tread of a cap/base construction, which tread or tread cap, as the case may be, is normally the outer rubber surface of the tire intended to be ground-contacting.
Thus, in one aspect, it is considered herein that the carbon black reinforced rubber of the tire carcass and associated tread normally provide a sufficiently low electrical resistance path to dissipate potential electrical energy and, thereby, retard or eliminate static electrical charge from building up and/or accumulating under dynamic conditions of a rotating tire on a vehicle traveling across the ground.
Alternatively, in practice, carbon black reinforced rubber tires may sometimes be prepared which have outer rubber treads designed to be ground-contacting which are quantitatively reinforced with silica or other non electrical conductive fillers and, thus, contain only minimal amounts such as, for example, 15 phr or less, or sometimes even less than 10 phr, of carbon black.
In such silica reinforced tire tread construction, although the various other rubber components of the tire, namely, the aforesaid overall tire carcass, are quantitatively reinforced with carbon black with a minimal amount, if any, of silica and, thus, may have a relatively low electrical resistance on the order of one megohm or less whereas, the silica reinforced tread itself may have a substantially high electrical resistance on an order of at least 20,000 megohms and, thus, such tread creates a degree of electrical insulating effect between the tire carcass and ground. Such a tire construction has a substantially less tendency to dissipate static electricity from the tire to the ground, and particularly from the metal rim of a tire/rim assembly to the outer surface of the tire tread and thence to the ground, which may be generated by a dynamic condition of the rotation of the tire on a moving vehicle. Accordingly, a potential for static electricity to build-up, or increase, is considered to be higher for such a tire construction with a silica reinforced tread than for a similar tire with a carbon black reinforced tread.
Therefore, it is desirable to provide a suitable path of relatively low electrical resistance between the tire bead portion and the tread outer surface for such a tire having a quantitative silica reinforced rubber tread and minimal, if any, carbon black reinforcement.
While the dissipation of generated electrical energy may not be completely understood, it is believed that, insofar as a vehicular tire is concerned, electricity may be transmitted primarily from the metal rim, of steel or aluminum, for example, thence on or through the carbon black reinforced rubber surface of the tire carcass to the outer surface of a carbon black reinforced rubber tread and thence to the ground.
It is recognized that a thin outer rubber coating, if applied to a tire tread surface, will relatively quickly wear away as the tire is used, leaving the coating on surfaces within the grooves in a tire tread which typically has a lug/groove design or configuration. Thereby, it is considered herein that only a very small portion of the coating, namely the thickness of the coating on the walls of the tire tread lugs, is actually available to be directly presented to, or contact, the ground to facilitate a relatively low electrical resistance from the tire to the ground for a tire with tread which is quantitatively reinforced with silica.
It is, therefore, considered herein that in order to be practical, the carbon black reinforced tread outer top cap layer over the outer surface of the silica reinforced rubber tread should be a thin carbon black-containing rubber layer integral with the tread and, thus, having good adhesion to the tread, particularly within the tire tread grooves including the lug walls, and present a sufficient cross-sectional thickness, or surface area, to the ground of the outer top cap layer on the tire lug walls to be effective after the said layer wears away from the outer surface of tread lugs.
In one alternative aspect, for a tire tread conventionally configured with a combination of lugs and grooves, it is desired that the grooves connect directly or indirectly with the carbon black reinforced rubber shoulder of the tire, the area of the tire where the sidewall and tread meet, in order for the carbon black reinforced outer cap layer to more fully connect with the carbon black reinforced rubber portion of the tire, namely, the tire carcass and including the tread base in the case of a tread cap/base construction.
In a tire tread cap/base-wing construction, which is well known to those having skill in such art, and for the purposes of this invention, it is envisioned that the cap is substantially silica reinforced with a minimal amount of carbon black and its base and wings are substantially carbon black reinforced.
In practice, it is desirable that the outer tread top cap layer, wings and tread base, if any, (i) contain a quantitative amount of carbon black and are of a relatively low electrical resistance to aid in dissipation of electrical energy under the aforesaid conditions, (ii) and are co-vulcanized with the rubber tire tread in order that they be integral with the tread and the walls of grooves of a tire tread configuration composed of lugs and grooves
It is expected that the outer cap rubber composition will wear off of the outer surface of tire tread lugs during use of the tire so that the cross-section, or thickness, of the outer cap layer on the lug walls is relied upon to present a path of relatively low electrical resistance from the tread to the ground.
As used herein, the terms "quantitatively reinforced with silica", "quantitative silica reinforced rubber" and the like are generally used in conjunction with a tire tread, and with a rubber tire tread cap, in a tread cap/base construction, which contains about 30 to about 100, sometimes preferably about 30 to about 90 phr, of silica, and which may also optionally contain carbon black in which the carbon black is present in not more than about 20 phr. Often it is preferred that the ratio of silica to carbon black is at least 2/1 and sometimes at least 10/1.
By the term "carbon black" reinforced, it is meant that the rubber components of the tire carcass rubber which are carbon black reinforced, contain a quantitative amount of carbon black reinforcement, normally at least 25 phr, and a minimal amount, if any, of silica and the weight ratio of carbon black to silica is at least 5/1.
While it is recognized that most carbon blacks are electrically conductive to some degree, and that most carbon blacks will make most rubber compositions electrically conductive, or at least to some degree if they contain enough of the carbon black, some carbon blacks are more electrically conductive than others. For convenience herein, by the term "electrically conductive carbon black" is meant a carbon black characterized by having a BET surface area of at least 30 gm/cm.sup.2. The BET surface area of carbon black is a well known technique of characterizing such surface area by a nitrogen absorption method.
The term "consistent, or continuous, rubber composition" where used herein is intended to mean that the rubber composition is, basically, of the same rubber composition throughout the designated rubber components. In particular, it is intended in the practice of this invention that the tread miniwings and tread outer cap layer are of the same rubber composition and are of a unitary, or continuous composition and component or construction. Thus, in one sense, the outer tread cap layer is simply an extension of the tread miniwings. In another aspect of the invention it is intended that the tread miniwings, tread outer cap layer and the tread base, when a tread base is used, are of the same rubber composition and are of unitary construction. By the term unitary construction it is intended to mean that all of such tire tread components are joined together in a single, unitary construction.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber". In the description herein, rubber and elastomer are used interchangeably.
In the description herein, the term "vulcanized" or "vulcanizable", may, on occasion, be used interchangeably with the terms "cured" and "curable".